The invention relates to a process for the preparation of copolymers of carbon monoxide with one or more compounds containing an ethylenically unsaturated bond.
Linear copolymers in which units originating from carbon monoxide substantially alternate with units originating from ethylenically unsaturated compounds, have been extensively described in the patent literature. Convenient methods for the preparation of these copolymers are disclosed in, inter alia, EP 181014 and EP 248483.
In these and similar preparation methods the monomers, i.e. carbon monoxide and one or more ethylenically unsaturated compounds, are contacted with a suitable catalyst, usually a catalyst system based on a metal of Group VIII of the Periodic Table.
The copolymers obtained typically have an average molecular weight, calculated as number average (Mn), of more than 10,000, and have established utility as thermoplastics. They may be processed by means of conventional techniques into films, sheets, plates, fibers, shaped articles such as containers for foods and drinks, and parts for the car industry.
For obtaining copolymers with optimal properties for the intended use, it is advantageous to minimize variations in the average molecular weight of the copolymers produced. Therefore, it would be desirable to perform the copolymerization process under such conditions that the average molecular weight of the product can be controlled and substantially remains at the desired value.
For other uses of thermoplastic polymers other than those previously mentioned, it is preferred to utilize polymers having considerably lower average molecular weights, e.g. of 2500 or lower. These polymers, such as oligomers and low molecular weight copolymers, may be used as such, or may be applied as starting materials for the preparation of other valuable products, e.g. as intermediates for the production of plastics, as blending components or as plasticizers for other polymers. It would therefore also be of advantage if the polymerization process could be carried out under such conditions that copolymers with lower molecular weight are predominantly formed.
It has already been proposed to subject the copolymers with an average molecular weight of more than 10,000 to a fractionation treatment and to separate the low molecular fraction therefrom. This method has the disadvantage of being time-consuming and moreover, the yield of the desired low molecular fraction is generally very low. It has further been proposed to increase the temperature at which the copolymerization is carried out. Although the formation of products with lower molecular weight is thus enhanced, the stability of the catalyst system often becomes a problem.
The molecular weight of the copolymers produced can also be reduced by addition of molecular hydrogen during the reaction. It appears that with most catalyst systems a significant reduction in molecular weight only occurs by applying large amounts of hydrogen, say of 60 mol % or more, based on carbon monoxide. Apart from the unattractiveness of a high-hydrogen consumption, the use of large quantities of hydrogen moreover often results in a reduction of the metal of Group VIII, accompanied by inactivation of the catalyst system. Only in exceptional cases, by selection of specific ligands in the catalyst system, the desired reduction in molecular weight of the copolymers can be achieved, without impairing the activity of the catalyst system.
Surprisingly, it has now been found that an adequate control of the average molecular weight of the formed copolymers can be achieved and that products having a relatively low molecular weight can be prepared, when the copolymerization reaction is carried out in the presence of a minor amount of a compound containing a hydride moiety.